Abstract: High-fill slope engineering is common in the construction of airports in plateau mountainous areas of China. Influenced by the plateau climate and complex topography, the stability and failure mechanisms of such high-fill slopes are complicated, especially under adverse conditions such as extreme rainfall. The interaction between geomaterials and retaining structures exhibits strong nonlinearity. This study aims to develop an effective approach for accurately analyzing the stability and failure mechanisms of such slopes. The material point method (MPM) high-performance computing software CoSim-MPM was employed, and the strength reduction method (SRM) was incoporated to evaluate slope stability and investigate the failure process. Taking a high-fill retaining wall slope at a mountainous airport as a case study, a systematic analysis was conducted on its stability and failure mechanisms under natural and heavy rainfall conditions. The results show that the slope remains generally stable under both natural and rainfall conditions; however, the failure modes differ. Under natural conditions, the slope exhibits a push-type failure, while under heavy rainfall conditions, it shows a traction-type failure. The findings demonstrate that the proposed approach offers significant advantages for studying the stability and failure mechanisms of high-fill slopes, providing a scientific basis for the site selection, design, and risk assessment of similar engineering projects.